The term electroscope is given to instruments
which serve two primary purposes: 1) to determine if a body is
electrified, and 2) to determine the nature of the electrification. An
electrometer, on the other hand, is a specialized form of
electroscope that includes a calibrated scale for reading the strength
of the charge.*

Fig. 1
Cavallo Pocket Electroscope16

The first electroscope was a device called a
versorium, developed in 1600 by William
Gilbert (1544-1603), Physician to Queen Elisabeth I.. The
versorium was simply a metal needle allowed to pivot freely on a
pedestal. The metal would be attracted to charged bodies brought near.

A simple hanging thread - called a
"Pendulous thread" by Stephen Gray,
(1666-1736) was
introduced around 1731. The thread would be attracted to any
electrified body nearby. In 1753 John Canton improved the
electroscope by adding two small pith balls suspended by fine linen
thread. The upper ends of the threads were fastened inside a wooden box.
When placed in the presence of a charged body, the two balls would
become similarly charged, and since like charges repel, the balls would
separate. The degree of separation was a rough indicator of the amount
of charge.

In 1770, William Henley
developed the first portable quadrant electrometer. The date is sometimes referenced as
1772 since that was the first time the invention was published (phil
trans 1772, 63, p. 359.) The
device consisted of an insulated stem with an ivory or brass quadrant
scale attached. A light rod or straw extended from the center of the
arc, terminating in a pith ball which hung touching the brass base of the
electrometer. When the brass was electrified the ball would move away
from the base, producing an angle which could be read off of the scale.
See illustration, below left.

Henley Electrometer

Gilbert's Versorium

In 1779 Tiberius Cavallo (1749-1809) designed an improved
electroscope, his "pocket electrometer" (see figure 1). The device
included several improvements including, for the first time, enclosing
the strings and corks inside a glass enclosure to reduce the effect of
air currents.16

The first true electrometer came from Horace
Benedict de Saussure (1740-1799) who placed the strings and balls inside an inverted
glass jar and added a printed scale so that the distance or angle
between the balls could be measured. It was de Saussure who discovered
the distance between the balls was not linearly related to the amount of
charge. However, The exact "inverse square" relationship would be left for
Charles Coulomb to discover in 1784.)12

Abraham Bennet (1750 - 1799) discovered that gold
foil was much more sensitive than cork or pith and created the first
gold leaf electroscope in 1786.12

*I should point out that these distinctions are a
modern construction; the 18th century authors referred to their simple
string devices as electrometers.

Below are some of the electroscopes and electrometers in my collection:

German
Quadrant Electroscope
c. 1900

Henley Electrometerc. 18th century

Pith-ball Demonstration

While not
technically an electroscope, the device does indicate the presence of
electric charge by rapid movement of the pith-balls inside the glass.